Swift 5.1 support

Use stdlib Result instead of TAsyncResult

Fix: deprecations for `withUnsafeBytes`

Bump CI job Xcode version

Hash (into)

Co-authored-by: Sophie Lambrakis <SLambrakis@users.noreply.github.com>
Co-authored-by: Alexander Edge <alex@alexedge.co.uk>
diff --git a/.travis.yml b/.travis.yml
index 400cfa1..f3dc7e4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -152,22 +152,11 @@
     # ------------------------- phase: swift ------------------------
     # We lint the podspec
     - os: osx
-      osx_image: xcode10.1
+      osx_image: xcode11.3
       language: swift
       script:
         - gem update cocoapods
-        - pod lib lint --allow-warnings --swift-version=3
-      env:
-        - JOB="pod lib lint"
-
-    - os: osx
-      osx_image: xcode10.2
-      language: swift
-      script:
-        - gem update cocoapods
-        - pod lib lint --allow-warnings --swift-version=4.0
-        - pod lib lint --allow-warnings --swift-version=4.2
-        - pod lib lint --allow-warnings --swift-version=5.0
+        - pod lib lint --allow-warnings --swift-version=5.1
       env:
         - JOB="pod lib lint"